Polyimide resins generally have excellent heat resistance, but are insoluble in commonly used solvents other than certain high.boiling organic solvents. In order to utilize polyimide resins as coating material for electronic parts or the like, a special method for forming a polyimide resin film must be employed involving dissolving a polyamic acid which is a polyimide precursor in an organic solvent, applying the solution to a substrate to form a thin coating, and heating the coating at elevated temperatures for an extended period of time, thereby curing the coating into a polyimide film. More particularly, such a polyimide resin film is most often prepared by effecting addition reaction of a tetracarboxylic acid dianhydride and an aromatic diamine in an organic solvent to form a polyamic acid or a polyimide precursor. The reaction product is applied to substrates or articles, for example, electronic parts as a thin coating. The coated substrates are heat treated at elevated temperatures of at least 300.degree. C. for an extended period of time, thereby achieving imidization into a polyimide film through dehydration.
This method, however, suffers from several drawbacks. Heating at elevated temperatures for an extended period of time is disadvantageous as a commercial process, especially from the standpoint of energy saving. If heating is insufficient, the resulting polyimide resin will have polyamic acid left therein which later causes degradation of the polyimide resin, often inviting humidity and corrosion resistance losses. Particularly when the polyimide resin film is applied to electronic parts as an insulating protective film, such a degraded resin gives rise to serious problems including deterioration of electronic parts and reduced lifetime.
One possible approach for overcoming these problems is the use of a polyimide resin soluble in an organic solvent. The resin solution is applied to a substrate. The coated substrate is heated to evaporate off the solvent, leaving a resin film.
Several methods have been proposed for preparing such organic solvent soluble polyimide resins. For example, Japanese Patent Publication No. 26878/1972, Japanese Patent Application Kokai Nos. 65227/1980, 187430/1983, 35026/1985, and 197731/1985 disclose that polyimide resins soluble in phenolic solvents such as phenol and halogenated phenols are prepared by heating a tetracarboxylic acid dianhydride and an aromatic diamine in such a solvent. Japanese Patent Publication No. 30319/1977 and Japanese Patent Application Kokai Nos. 83228/1986 and 18426/1987 disclose that polyimide resins soluble in polar solvents such as N.methyl-2-pyrrolidone are prepared using a specific tetracarboxylic acid dianhydride and a specific diamine.
These methods also have several drawbacks. When it is desired to use the polyimide resin prepared by the former method, the resin is dissolved in a phenolic solvent to form a coating composition which generates an irritative odor like cresol odor upon evaporation of the solvent. Contact of the solvent to the skin causes chemical burn. Thus this coating composition requires careful handling and is less safe or hygienic. Where the polyimide resin prepared by the latter method is used as a solution in a polar solvent such as N-methyl-2-pyrrolidone, a diamine such as toluene diamine which disorders the crystallinity of the resin must be used to render the resin soluble at the sacrifice of electrical properties and toughness inherent to the polyimide resin. Polar organic solvents such as N-methyl-2-pyrrolidone have such a high boiling point that a heat treatment must be carried out at elevated temperatures for an extended period of time in order to fully remove the solvent at the sacrifice of operation efficiency.